The invention relates to an observation instrument comprising a hollow shaft having a midplane and in whose distal end region there is arranged an optoelectronic imaging system which has on the image entrance side a distal deflection prism whose deflection faces lead an image into an optical lens system which images the image onto the image plane of an image sensor arranged proximally from the lens system, the image sensor having an active region which is situated asymmetrically relative to the outer contour of the image sensor.
Such an observation instrument in the form of a medical endoscope is known, for example, from U.S. Pat. No. 4,720,178 B2.
It is known from U.S. Pat. No. 4,809,680 that image sensors have an active region which is situated asymmetrically relative to the outer contour of the image sensor.
Image sensors usually exhibit a quadrangular, mostly a rectangular outer contour. For design reasons, in particular in order to accommodate the numerous electronic elements, the active region is situated asymmetrically, that is to say mostly offset laterally outwards. The active region likewise mostly has a rectangular contour.
Depending on the size ratios and the structural conditions within the shaft, such an image sensor can be installed vertically or tilted or horizontally.
If it is installed vertically, its image plane runs perpendicular to the longitudinal axis of the shaft. In tilted arrangements, said image plane is somewhat tilted to said longitudinal axis. Given horizontal installation, the image plane of the image sensor runs in the direction of the longitudinal axis and mostly in a fashion offset laterally parallel thereto.
Optimum utilization of the image information requires the image to be imaged onto the active region of the image sensor such that the image is situated within said active region. This is illustrated in FIG. 1. It is to be seen there that an image sensor 16 with an asymmetric active region 18 is installed vertically in a shaft 10. If the image area 20 is intended to be optimally detected by the active region 18, said image area mostly being circular, the main beam 21 of the optical path is situated in a fashion laterally offset from the middle longitudinal axis 14 of the shaft 10, and thus also in a fashion laterally offset from the midplane 12 thereof. This would also bring about the existence of an asymmetric sight cone of the observation instrument.
An asymmetric sight cone 22 as illustrated by way of example in FIG. 3 has certain disadvantages, however. Illustrated in FIG. 3 is a shaft 10 of an endoscope over which an overshaft 24 is pushed. Protruding from the distal end of the overshaft 24 are two rod-shaped elements 26 and 28 which are connected at the distal end via a resectoscope loop 30. In an asymmetric sight cone 22, because of the asymmetry of the sight cone 22 the observer sees two points 27 and 29, axially offset from one another, of the bars 26 and 28. This is undesirable, because the observer then sees the positions or, when the shaft 10 is shifted further in the distal direction, the resectoscope loop 30, not simultaneously or uniformly or out-of-focus on one side. Consequently, there is a desire for symmetrical sight cones 22, as is shown in FIG. 4. The viewpoints 27′ and 29′ are then situated at the same axial height in this stage.
However, as illustrated in FIG. 2, this can have the consequence that the image area 20 is no longer situated with its volume entirely in the active region 18 of the asymmetric image sensor 16 given a symmetrically designed imaging optics.
It is therefore an object of the present invention to provide an observation instrument in the case of which there is a sight cone which is as symmetrical as possible and the image is situated as completely as possible in the active region of the image sensor in the case of asymmetric image sensors.